It's pretty obvious that Doctor Who's going to be one of the most expensive shows the BBC produce, and I shouldn't think the same amount of money is there as it was in 2005-2009.
I'd imagine that is the reason, and specifically, that BBC Wales's budget has been cut (like all the rest of the BBC), so they can't afford to make all the series they like, so they keep pushing Doctor Who back to split its costs over two financial years (regardless of what BBC1, Worldwide, etc, would prefer).
What's frustrating is the contempt for the viewers of not simply admitting that - or whatever the explanation actually is.
It's not as if it's limited to Who - tune into Feedback on Radio Four, and every couple of weeks you'll hear a criticised BBC exec whose answer comes down to "Because I say so, and my opinion must be more important than yours because I've got the job and you haven't, so just shut up and watch/listen to what I'm good enough to give you," - but it is irritating. Because we're not idiots who can't handle the truth - well, not all of us.